Introduce a new `passwords::hash_with_salt_async` and change the return
type of `passwords::check` to `future<bool>`. This enables yielding
during password computations later in this patch series.
The old method, `hash_with_salt`, is marked as deprecated because
new code should use the new `hash_with_salt_async` function.
We are not removing `hash_with_salt` now to reduce the regression risk
of changing the hashing implementation—at least the methods that change
persistent hashes (CREATE, ALTER) will continue to use the old hashing
method. However, in the future, `hash_with_salt` should be entirely
removed.
Refs: scylladb/scylladb#26859
4ffdb0721f